Conventional ultrasound scanners create two-dimensional B-mode images of tissue in which the brightness of a pixel is based on the intensity of the echo return. Alternatively, in a color flow imaging mode, the movement of fluid (e.g., blood) or tissue can be imaged. Measurement of blood flow in the heart and vessels using the Doppler effect is well known. The phase shift of backscattered ultrasound waves may be used to measure the velocity of the backscatterers from tissue or blood. The Doppler shift may be displayed using different colors to represent speed and direction of flow. In power Doppler imaging, the power contained in the returned Doppler signal is displayed.
Ultrasound imaging is also useful in positioning an instrument at a desired location inside a human body. For example, in order to perform a biopsy on a tissue sample, a biopsy needle must be accurately positioned so that the tip of the biopsy needle penetrates the tissue to be sampled. By tracking the biopsy needle using an ultrasound imaging system, the biopsy needle can be directed toward the target tissue and inserted to the required depth. Thus, by visualizing both the tissue to be sampled and the penetrating instrument, accurate placement of the instrument relative to the tissue can be achieved.
However, in order to visualize the biopsy needle most effectively, it is important to ensure that the incident ultrasound beam is perpendicular with respect to the biopsy needle. The smaller the angle at which the biopsy needle is inserted relative to the axis of the transducer array, i.e., the imaginary line normal to the face of the transducer array, the more difficult it becomes to visualize the needle. A needle is a specular reflector, meaning that it behaves like a mirror with regard to the ultrasound waves reflected off of it. The ultrasound is reflected away from the needle at an angle equal to the angle between the transmitted ultrasound beam and the needle. In a typical biopsy procedure using a linear probe, the geometry is such that most of the transmitted ultrasound energy is reflected away from the transducer array face and thus is poorly detected by the ultrasound imaging system.
Commonly, biopsy needles are inserted into anatomy to obtain a tissue sample. The tissue sample may be in the form of a cyst or tumor mass that needs to be investigated. However, due to the thin and flexible nature of the biopsy needle, sometimes the needle deflects off of the cyst or tumor. This deflection brings into question the validity of the tissue sample.
Thus there is a need for a technique for improving visualization of a needle-like instrument being inserted into a human body so that the position of the needle tip with respect to the surrounding anatomy is clearly shown.